Heterosperma pinnatum plants produce a range of different seed types from the same head, as a consequence of floret type and position. These seeds have different dispersal and dormancy properties. Photo by Larry Venable.

Individuals within populations vary in their demographic properties, not only because of widely-studied factors like age, size, and sex, but because they experience different environments, because their maternal environments vary, and because they vary genetically (and because these latter three factors can all interact). Populations are heterogeneous, but most of the models we use in ecology and conservation biology treat them as though they were monomorphic.

Research in our lab, in collaboration with Bruce Kendall and Joe Stover (both at UC Santa Barbara), has shown that problems can sometimes result. In particular, we have shown treating populations as though they are homogeneous can seriously misestimate risk of extinction as well as rates of population growth. We are presently analyzing a number of empirical data sets on the size of this effect. These analyses will also be useful in demonstrating appropriate statistical methods for studying within-population heterogeneity.

Heterogeneity turns out to play a big role in other aspects of demography; in a forthcoming paper we will show that habitat heterogeneity leads to biased estimates of vital rates and generates spurious correlations among the vital rates.